1. Field of the Invention
The present invention relates to a linear frequency-modulated pulse radar in which the echo signal is mixed with a de-ramp signal, in order to reduce the bandwidth.
2. The Prior Art
The de-ramping method is also frequently referred to in the technical literature as the de-chirp method. This method is usually used in broadband pulse radar systems with linear frequency modulation (chirp signal), in order to reduce the bandwidth of the echo signal in the IF plane as described in Carrara, Goodman, Majewski: “Spotlight Synthetic Aperture Radar,” Artech House, 1995, pages 26-31.
FIG. 1 illustrates the de-ramping method. When using the de-ramping method, a de-ramp signal is generated, which has the same chirp rate (bandwidth produced per time unit—corresponds to the incline in the frequency/time diagram) as the transmission signal; however, the de-ramp signal has a longer pulse duration and oscillates at a different mean frequency. The pulse duration of the de-ramp signal is selected so that the echo signals of the entire observation range are covered. This de-ramp signal is fed into the first downward mixer as the LO signal, with a delay. The resulting IF signal with an echo signal from a point target within the observation range is a non-modulated pulse (signal having fixed frequency and limited duration). The frequency of this pulse is dependent on the difference frequency between the transmission signal and the de-ramp signal, and on the running time of the echo signal. In FIG. 1, the first (echo signal 1) and the last (echo signal 2) of the echo signals received from the predetermined observation range are plotted. If the length and the delay of the de-ramp signal are adjusted accordingly, the bandwidth of all the echo signals from the target range is now determined only by the transmission signal shape and by the radial depth of the observation range. As is evident from FIG. 1, top and bottom diagram, the bandwidths of the echo signals mixed with the de-ramp signal are less than the bandwidth of the transmission signal.
Generation of the de-ramp signal takes place, according to the state of the art, in accordance with the structure described in FIG. 2. The transmission signal is first applied at the output of the signal generator SG. This signal is usually a pulse having a linear frequency increase within the pulse width (chirp signal).
The carrier frequency of this signal is an intermediate frequency IF1 predetermined by the system. The IF1 transmission signal is converted by means of the reference oscillator LOs, to the transmission frequency fs (first frequency processing channel) and emitted. The carrier frequencies (mean frequencies) of the frequency-modulated signal are shown in parentheses in the figures, at the line segments in question.
For the reception case, with a predetermined location of the observation range, the de-ramp signal is generated in the signal generator SG, with the same chirp rate and frequency as the transmission signal, but with a greater pulse width. In this way, the entire observation range is covered. The conversion of this de-ramp signal to a second intermediate frequency IF2 takes place by way of the reference oscillator LOdr (second frequency processing channel).
In the reception case, the echo signal (carrier frequency fs) is mixed with the de-ramp signal to a reception intermediate frequency IF, which is usually identical with IF1. Mixing to the base band takes place by way of the reference oscillator LObb. The base band can also contain a frequency offset, depending on the bandwidth and the possible sampling rate of the A/D converter.
In WO 2004/005961 A1, another frequency-modulated pulse radar is described, whereby the transmission signal and the de-ramp signal are generated with the same reference oscillator.